Proposed is a multi-level study is to determine the association between features of the urban environment and sexual and drug use risk factors for HIV infection. Although there are at present several multi-site intervention studies that aim to minimize individual high risk sex and drug-use behavior, surprisingly little systematic research has explored the relation between features of the urban environment, HIV risk behavior, and disease morbidity (sexually transmitted diseases and HIV) while controlling for relevant individual-level covariates. This study will examine the independent and interactive effect of features of the urban social environment (e.g., income distribution, neighborhood disadvantage, racial segregation) and the physical environment (e.g., population density, public transportation, built environment) as they relate to sexual and drug use risk factors for HIV infection and to HIV and STD prevalence. To address the study aims, we will recruit 2,000 persons (including current drug users, past drug users, and never drug users) from twenty-four New York City neighborhoods over a four year period. We will define neighborhoods using cluster analysis, ethnographic mapping, and community consultation. Participants will be recruited using a number of street outreach techniques. Individual risk behaviors and potential mediators and moderators will beassessed through in-person interviews. Features of the urban environment will be assessed from the ethnographic assessments and using publicly available archival information (e.g., New York City Housing andVacancy Survey, Mayor's Management Report). Hierarchical multivariate modeling will be used to determine the relations between features of the urban environment and individual risk behavior, HIV, and STDs. Identification of the key features of the urban environment that shape risk behavior can guide structural interventions, increase the efficiency of individual-level behavioral interventions, and identify features of urban neighborhoods that could suggest population vulnerability to rapid transmission of HIV.